As more and more spacecraft and communications systems rely on the Global Positioning System (GPS) for their timing source, it is clear that the reliability of the spacecraft or communications link is a function of the entire GPS system. Even though GPS has demonstrated superb reliability and availability, a few operational glitches have occurred that affected the time and position solutions (i.e. the time and position output). At least one of these has contributed to a complete shutdown of a prototype TDMA system until the problem was cleared by the GPS Control Segment some 30 minutes later (the affected satellite was set unhealthy). A major and somewhat unadvertised flaw in the GPS system design is that there is no apparent way for the Control Segment to immediately remove a bad satellite from the constellation, the process time to do so is typically between 5 and 45 minutes.
Many spacecraft orbiting the earth use information derived from GPS for navigation and/or to maintain them in their specific orbit. A spacecraft using GPS for navigation includes a GPS receiver with a plurality of channels (12 or more), each dedicated to tracking a different GPS satellite. Generally, the spacecraft is moving in an orbit or path different than the satellites in the GPS constellation which means that the GPS satellites being tracked are continuously changing
The detection of failed GPS satellites is generally limited by the presence of Selective Availablity (SA). As is well known by experienced GPS users, SA, imposed on civil users by the Department of Defense, severely limits navigation accuracy. In addition, it also limits conventional methods of failure detection as, for example, referenced in Brown, "A Baseline RAIM Scheme and a Note on the Equivalence of Three RAIM Methods", Journal of the Institute of Navigation, Fall 1992. The fundamental limitation imposed by SA limits failure detection to error magnitudes which are large in comparison to SA. However, information provided by a failed or failing GPS satellite can seriously affect the navigation of a spacecraft relying on GPS information.
In U. S. Pat. No. 5,583,774, issued Dec. 10, 1996 and entitled "Assured-Integrity Monitored-Extrapolation Navigation Device", an integrity monitor is developed for GPS using sensor augmentation. In this device, an Inertial Navigation System (INS) is used to maintain a highly accurate trajectory for a period of time sufficiently long (e.g. 30 minutes) to enable isolation of failed satellites by the GPS Control Segment. A bank of Kalman filters, using the GPS-calibrated INS as a reference trajectory, is used to estimate a range bias for each satellite, which is used to confirm its health. Only satellites passing this health test can be used in the calibration. The major problem with this system is the need for external sensors and banks of Kalman filters, which greatly increases the size and complexity of the system.
Accordingly, it is highly desirable to provide a method and apparatus which is capable of quickly and reliably isolating and removing failed or failing GPS satellites from the GPS position and time calculations used in navigation of a spacecraft.